Conventionally, as a two-way optical communication apparatus, the following two-way optical communication links have been known: a single-mode optical fiber which transmits single-mode light and serves as a transmitting medium, and a multimode optical fiber which transmits multimode light and serves as a transmitting medium.
An example of the single-mode optical fiber is a quartz glass optical fiber whose core is made of quartz glass. A loss caused by the quartz glass optical fiber is so small that transmission is possible over a long distance at high speed. The quartz glass optical fiber is coupled to an optical transmit/receive module(two-way optical communication device) so as to be widely used for the two-way optical communication link such as a LAN that is based on an ATM(asynchronous transmission mode).
However, the cost of the quartz glass optical fiber is high, the single-mode optical fiber needs to have a small diameter of merely several .mu.m in view of a problem on manufacturing, and furthermore, it is difficult and time-consuming to adjust a coupling to the optical transmit/receive module, resulting in an increase in cost. Consequently, it is difficult to adopt the quartz glass optical fiber for a small-scale network such as a home network.
Meanwhile, examples of a multimode optical fiber are fibers including the quartz glass optical fiber and a plastic optical fiber(hereinafter, abbreviated as POF) whose core is made of plastic. Under the present circumstances, it is difficult for the POF to make a transmission over a long distance because of its relatively great transmission loss; however, the materials are inexpensive, the bending loss is small, resistance to cracking is offered, and a fiber with a large diameter of approximately 1 mm can be easily manufactured. Therefore, the POF makes it easy to adjust the coupling to the optical transmit/receive module and to reduce the cost of installing; consequently, the POF is suitable for a small-scale network such as a home network.
FIG. 22 illustrates an example of the two-way optical communication link which includes the POF serving as a medium. Here, two POFs 102 are respectively provided for transmission and reception. As a light-emitting element 107 on the transmitting side, an LED or a semiconductor laser is adopted and is coupled to the POF 102 directly or via a lens. on the receiving side, a photodiode is used as a light-receiving element 106 so as to receive light transmitted from the POF 102.
Such a two-way optical communication link has an advantage of easily adjusting the light-receiving elements 106 and the light-emitting elements 107 with the POFs 102 by making use of the large core diameters of the POFs 102. However, this arrangement requires two POFs 102, thereby increasing the cost in the case of transmission over a long distance.
Further, Japanese Laid-Open Patent Publication No. 191543/1983 (Tokukaisho 58-191543) discloses an optical transmit/receive module which is capable of two-way communication by using one optical fiber. As shown in FIG. 23, the optical transmit/receive module has a construction in which (a)a light-emitting element 207 which has a round light-emitting surface for launching light into an optical fiber 202 and (b) a ring-shaped light-receiving element 206 for receiving light incident from the optical fiber 202 are concentrically formed, an insulating space 210 being provided therebetween.
With the above-mentioned arrangement, upon transmitting, light emitted from the light-emitting element 207 is directly transmitted to the optical fiber 202, and upon receiving, light incident from the optical fiber 202 is received by the light-receiving element 206; therefore, it is possible to transmit and receive light merely with one optical fiber 202.
However, the optical transmit/receive module which is provided with the light-emitting element 207 at the center of the light-receiving element 206 causes the following problems: the light-emitting element 207 or the light-receiving element 206 is adversely affected due to heat of the light-emitting element 207, and for example, a stray light, which is transmitted from the light-emitting element 207 and is reflected on the incident surface of the optical fiber 202, may easily enter the light-receiving element 206, resulting in degradation in sensitivity to reception.
Moreover, the above-mentioned optical transmit/receive module requires a face-emitting type of the light-emitting element 207 due to a structural constraint; however, in the case of a face-emitting type of the LED, it is difficult to increase the speed. Furthermore, with regard to the semiconductive laser which is capable of increasing the speed, the face-emitting type has not been put into practical use; therefore, this arrangement offers drawbacks with reliability and cost.
Further, as shown in FIG. 24, a method in which a half mirror 310 separately handles transmitted light and received light has been known. With this method, by changing the incident angle with the half mirror 310, light emitted from a light-emitting element 307 enters into an optical fiber 302, and light incident from the optical fiber 302 passes through the half mirror 310 and is received on a light-receiving element 306; therefore, it is possible to transmit and receive light merely with one optical fiber 302.
However, in the method in which the half mirror 310 separately handles transmitted light and received light, with regard to the transmitted light and the received light, a loss of approximately 3dB occurs on the half mirror 310 and it is difficult to adjust an optical axis; consequently, this method tends to decrease the reliability on transmission and reception of light.
In addition to the aforementioned methods, another method which transmits and receives light by using one optical fiber adopts an optical branch path of an optical waveguide. For the optical waveguide, materials such as glass, a semiconductor, and a plastic are now under study. Thanks to its small loss, a glass optical waveguide is used for the optical transmit/receive module with the single-mode optical fiber serving as the transmitting medium. Further, as disclosed in Japanese Laid-Open Patent Publication No. 188402/1991 (Tokukaihei 3-188402), a plastic optical waveguide can be easily worked and can be handled in a relatively simple way, thereby receiving attention as a substitute for the glass optical waveguide.
However, it is difficult to work on a thick film of the glass optical waveguide; thus, in the case when the glass optical waveguide is coupled to the multimode optical fiber such as the POF with a large diameter, the coupling loss increases. Moreover, with regard to the plastic optical waveguide which is available for the optical transmit/receive module being appropriate for coupling to the multimode optical fiber, no publication has been disclosed yet.
Meanwhile, Japanese Laid-Open Patent Publication No. 334644/1996 (Tokukaihei 8-334644) discloses an optical branch device made of plastic. As shown in FIG. 25, the optical branch device is constituted by a clad 410 which is composed of a single molded body made of resin, and a core 403 which branches into not less than two paths in the clad 410. In the optical branch device, the end of the core 403 is coupled to the POF so that light incident from the POF propagates through the core 403 and branches off a branching portion 414 before having been launched.
Such a plastic optical branch device makes it easy to form the core 403 having the same size as the core diameter of the POF and provides a high-efficiency coupling to the multimode optical fiber such as the POF that has a large core diameter.
However, in such a plastic optical branch device, the incident light is divided into virtually equal amounts at the branching portion 414 so that in the case when the plastic optical branch device is used as the optical transmit/receive module, the amount of received light is reduced in half. Therefore, this arrangement causes degradation in quality for reproducing a signal from the received light and a reduction in the reliability. In addition, the core 403 and the clad 410 are formed by using a molding operation; thus, it is difficult to achieve an integration with the light-receiving element and the light-emitting element, and to design a smaller version.